User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:15.0) Gecko/15.0 Firefox/15.0a1 Build ID: 20120528093039 Steps to reproduce: I use Firefox Nightly. I was on a slow internet connection and noticed a full 40mb update was being downloaded. I quit Firefox. Two days later I started Firefox again. Actual results: The partially downloaded update was completed and I restarted Firefox to apply the update. Immediately afterwards I noticed another full 40mb update being downloaded and I had to restart Firefox once more. Expected results: The update machinery should have checked if there was a later update available, before completing the in-progress one. If a later update is available, it should cancel the current update and get the latest version instead. Nightly users are the most likely to experience this, but I've had the same happen to me with stable builds on servers and security updates, where I'd use Firefox only once a month or less. Of course this isn't a high priority at all, just saving some bandwidth and potentially getting updates to be used earlier (one Firefox restart cycle earlier).

The same problem occurs if you've already downloaded but not yet applied an update, and there's a newer one available. There's currently no way to skip the downloaded update and instead download and apply the *latest* update. The about dialog already checks for updates when there's none downloaded. Can it just check for updates even if there's a downloaded update cached? If it detects a newer update available, throw away what you've got cached and download the newest update instead. Otherwise, show the "apply update" button.